This invention relates to the field of joining metals by solid state bonding (diffusion bonding) and particularly to joining aluminum alloys, magnesium alloys, or composites having these alloys as matrices.
Solid state bonding, or diffusion bonding, is the metallurgical joining of similar or dissimilar metals by applying heat and pressure for a time sufficient to cause commingling of atoms at the joint interface. The bonding is accomplished without any melting at a temperature which is at or above about one-half the absolute melting temperature of the metal being joined.
Structures have been produced successfully by diffusion bonding titanium alloys using methods as described in U.S. Pat. Nos. 3,920,175 and 3,927,817. Titanium is unique in that it can dissolve its own oxide at the diffusion bonding temperature. Thus, commingling of the metal atoms to form a diffusion bond can proceed without being blocked by the oxide which naturally forms on most metal surfaces. As illustrated in the above mentioned patents, diffusion bonding has been combined with superplastic forming to produce many high quality monolithic aerospace structures.
Unfortunately, the oxide which occurs on aluminum and magnesium alloys is not dissolved into the base metal at diffusion bonding temperatures. These alloys have a high affinity for oxygen and an oxide film is ever present on their surface. If the oxide film is broken under atmospheric conditions, a new oxide layer is formed almost immediately.
Several approaches have been tried to diffusion bond aluminum. In one approach, the oxide is completely removed and the oxide-free surface is kept in a vacuum to prevent reformation of the oxide during bonding. U.S. Pat. No. 4,483,478 describes such a method whereby "sputtering" is used to remove the oxide and a vacuum is maintained during bonding. However, an extremely good vacuum (less than 10.sup.-8 torr) is required to prevent oxide formation on aluminum within a short time. This level of vacuum is not easy to achieve and the need to physically manipulate workpieces inside a vacuum chamber increases the expense and limits the capability of the process.
In another prior art approach, thoroughly cleaned aluminum sheets are compressed (or rolled) together by about 60% strain (i.e., their surface area is increased 60% while in intimate contact). The broken oxide skin rolls into globules generating good metallic contact and a diffusion bond. Although some configurations can be made by this method, the large amount of deformation required is not suitable for many applications because the regions of desired bond location can shift considerably during joining.
Other attempts to bond aluminum use intermediate materials such as coatings or a layer of a bonding material between the parts. Frequently, the intermediate material itself is difficult to bond to the aluminum. If it has a low melting or softening temperature, the resulting bond will be suitable for subsequent superplastic forming which is done at an elevated temperature. Further, in high strength aluminum alloys having a composition close to their solid solubility limits, it is nearly impossible to diffuse the constituents of the intermediate layer back into the alloy itself. These approaches have not provided consistent high strength bonds.